The present invention relates to an apparatus and a method for maintaining braking force acting on the wheels of a vehicle that has been stopped through depression of a brake pedal even after the brake pedal is released from depression.
Generally, if the brake pedal of a vehicle is released from depression when the vehicle is stopped on an inclined surface such as a slope, braking force acting on each of the wheels quickly drops, causing undesired downward movement (sliding) of the vehicle along the inclined surface. This may prevent the driver from smoothly re-starting the vehicle. To avoid this, an apparatus and a method for maintaining braking force of a vehicle involving hill-hold control have been proposed (as described in, for example, Japanese Laid-Open Patent Publication No. 59-179439). Through the hill-hold control, undesired movement of the vehicle is suppressed by maintaining braking force acting on the wheels of a stopped vehicle even after the brake pedal is released from depression.
Specifically, even if the brake pedal is released after the vehicle has been stopped through depression of the brake pedal, the wheels of the vehicle are maintained in a locked state through the hill-hold control by the braking force maintaining apparatus of Japanese Laid-Open Patent Publication No. 59-179439. Through such control, unintended movement of the vehicle is prevented from occurring even when the brake pedal is released in a state in which the vehicle is held in a stopped state on the inclined surface, such as a slope surface, through depression of the brake pedal. The hill-hold control is ended when the accelerator pedal is depressed for re-starting the vehicle.
Further, if the surface on which the vehicle is stopped exhibits low friction coefficient μ (as in the case of a surface covered with snow) and is inclined exceeding a certain level, the vehicle may move in an unintended manner, causing yaw of the vehicle. In this case, if the vehicle does not have the braking force maintaining apparatus executing the hill-hold control, the wheels of the vehicle can be unlocked by releasing the brake pedal. This permits the driver of the vehicle to manipulate the steering wheel for correcting the posture of the vehicle.
However, if the vehicle has the braking force maintaining apparatus of Japanese Laid-Open Patent Publication No. 59-179439, which executes the hill-hold control, the wheels of the vehicle are held in a locked state even after release of the brake pedal intended to correct the posture of the vehicle. In this state, the wheels cannot grip the ground surface. In other words, as long as the wheels are held in the locked state, the posture of the vehicle cannot be corrected by manipulation of the steering wheel, complicating operation of the driver.